1. Field of the Invention
The present invention relates to an antilock brake control fluid pressure system, and more particularly to an antilock brake control fluid pressure system for use in an automobile for the purpose to secure a minimum brake distance while preventing, at the time of braking, the automobile from loosing its directional stability and steering capability due to locking of a wheel.
2. Description of the Prior Art
Generally in an antilock brake control fluid pressure system for use in an automobile and the like, locking of a wheel is detected based on a slip speed which is a difference between a moving speed of the automobile and a rotating speed of the wheel at the time of braking to control brake pressure, and upon detection of the locking of the wheel, the brake pressure is reduced to regain the wheel speed up to an appropriate slip speed so as to achieve a minimized braking distance while avoiding the locking of the wheel.
Conventionally, as an antilock brake control fluid pressure system of the above type, there has been a known technique as shown in FIGS. 7 and 8 (disclosed in the Japanese Patent Unexamined Laid-Open Publication No. HEI 1-297350). According to the above-known technique, there is provided an expansion chamber 12 for accommodating fluid which is released from brake pressure when antilock control is effected. According as the brake pressure reduces, a spool type flow valve 6 moves downward, and a first fluid inlet 7 formed in a modulator 4 is closed to cut off supply of fluid from the master cylinder 1 into the flow valve 6 by way of the first fluid inlet 7.
Synchronously with effecting antilock control as shown in FIG. 8, a pump 5 is activated by a motor M, and an output from the pump 5 diverges into two ways one of which returns to the master cylinder 1 by way of a throttle 10, and the other is supplied to the inside of the flow valve 6 by way of a second fluid inlet 8 under a metal-edge control. With the above-mentioned operations, the output from the pump 5 in an antilock control operation is input to the inside of the flow valve 6 by way of the second fluid inlet 8 to increase the brake pressure on one hand while the output is fed back to the master cylinder 1 by way of the throttle 10 on the other hand. Therefore, when the expansion chamber 12 is filled with sufficient fluid, pulsation from the pump 5 to the master cylinder 1 is restricted by the throttle 10 to reduce pump noise as well as the vibration of a brake pedal 13.
However, in the conventional antilock brake control fluid pressure system as shown in FIG. 8, practically the expansion chamber 12 is almost empty inside except for the time immediately after reducing brake pressure because the pumping capability of the pump 5 is so designed as to have a sufficient capacity with respect to the amount of fluid corresponding to brake pressure reduction. Therefore, even when the pump 5 is operated, no sufficient fluid is continuously supplied to the second fluid inlet 8 of the flow valve 6. Therefore, the flow valve 6 is possibly pushed back to its position before effecting antilock control as shown in FIG. 7 due to the reduction of fluid pressure inside the flow valve 6 even when antilock control is effected.
Consequently, the master cylinder 1 is communicated with the pump 5 by way of the first fluid inlet 7 and the second fluid inlet 8. When the pump 5 is operated to generate an output in the above condition, the output is transmitted as pulsation to the master cylinder 1 by way of the second fluid inlet 8 of the spool modulator 4, a fluid path in the flow valve 6, and the first fluid inlet 7. The pulsation is transmitted via the brake pedal 13 to a driver as an unpleasant vibration. The same unpleasant feeling is given to the driver when the vertical movement of the flow valve 6 is transmitted as a noise source from the modulator 4 to the brake pedal 13. For the above reasons, it has been impossible to sufficiently suppress the transmission of pulsation such as the pump noise to the brake pedal.
In order to supply sufficient fluid from the master cylinder 1 to the second fluid inlet 8, it can be considered to increase the bore of the throttle 10 interposed therebetween. In such a case, pulsation from the pump 5 is disadvantageously transmitted to the master cylinder 1 without being sufficiently suppressed by the throttle 10.